This invention relates to the field of hydraulic motors and has particular application to hydraulic motors which are connected for driving cooling fans for automotive engines of the internal combustion type. Such engines typically are supplied with a liquid coolant which is circulated through a radiator. As the coolant flows through the radiator, it gives up heat to the radiator surfaces, which in turn are cooled by flowing air. If the radiator is mounted in a moving vehicle, a certain amount of cooling air is naturally generated. However, natural flow is undependable and entirely inadequate in a modern vehicle. Therefore it is customary to employ a cooling fan for producing a forced flow of cooling air.
Radiator cooling fans are driven by the engine, either via direct mechanical connection or indirectly with the aid of a fan motor. While a variety of motor types are available for such purposes, hydraulic motors are particularly desirable due to the availability of a hydraulic fluid supply in most automobiles. However, automotive hydraulic fluid is generally supplied by a fixed displacement pump driven by a fixed ratio mechanical connection to the engine. This means that the rate of flow of hydraulic fluid and the speed of the cooling fan will vary in direct proportion to the engine speed. This is not a desirable result, because desired fan speeds vary over a considerably narrower range than the associated engine speeds.
It will be appreciated that the rotation of a cooling fan is opposed by a reaction torque due to aerodynamic drag which rises as the square of the rotational speed. This reaction torque is overcome by forces generated in the engine. The forces, so generated, pressurize the hydraulic fluid to a pressure which produces a driving torque that will balance the reaction torque, when applied across the projected area (work area) of a working surface positioned in a displacement chamber of the hydraulic motor. This causes a power drain upon the engine, which rises as the third power of the engine speed or fan speed. However, there is a practical limit on fan speed due to noise considerations, power drain and structural integrity of the fan.
Automotive engine speeds typically vary between about 600 rpm and 4,000 rpm, as the engine operation goes from idle to grade. This is a ratio of nearly 1:7. However, the fan speed requirement does not increase anywhere near that much. While specific fan speed requirements will vary widely with engine design, it has been found that the rotation speed at grade needs to be only about 1.5 to 2.0 times that at idle. Thus, if a fixed displacement hydraulic motor is designed to produce an ideal fan speed at idle, it will run several times faster than is necessary at grade. On the other hand, if the motor operates at the correct speed for grade, it will be unable to provide adequate cooling at idle. Heretofore the problem has been solved in one of two ways: (1) providing a variable displacement hydraulic pump, or (2) setting the work area of the motor for operation at idle and restricting the maximum permissible motor speed through use of a bypass line to divert hydraulic fluid not required for driving the fan. The first solution involves undesired complexity and expense, and the second wastes power. For a typical prior art fixed displacement motor, the wasted power has been found to be about 550 BTU per min. at an engine speed of 3050 rpm.